This invention relates to cooling loads, particularly superconducting loads in rotating reference frames.
The invention relates to cooling loads including superconducting components (e.g., a superconducting magnet).
Superconducting rotating machines such as motors and generators must be cooled such that the field structures of their rotors are in the superconducting state. The conventional approach to cooling rotor field coils is to immerse the rotor in a cryogenic liquid. For example, a rotor employing field coils made of high temperature superconducting materials might be immersed in liquid nitrogen. In this case, heat generated by or conducted into the rotor is absorbed by the cryogenic liquid which undergoes a phase change to the gaseous state. Consequently, the cryogenic liquid must be replenished on a continuing basis.
Another approach for cooling superconducting magnets is the cryogenic refrigerator or cryocooler. Cryocoolers are mechanical devices operating in one of several thermodynamic cycles such as the Gifford-McMahon cycle and the Stirling cycle. Cryocoolers have found application, for example, in cooling the stationary magnets in magnetic resonance imaging systems. Good cryocooler performance depends in large part on a design optimized for the actual conditions within which the cryocooler operates. More recently cryocoolers have been adapted for operation in rotating environments, such as superconducting motors and generators. One approach for doing so is described in U.S. Pat. No. 5,482,919, entitled xe2x80x9cSuperconducting Rotor,xe2x80x9d issued to Joshi, assigned to the assignee of the present invention, and incorporated herein by reference. An approach for cooling field windings in a superconducting motor is described in U.S. Pat. No. 5,848,532, entitled xe2x80x9cCooling System for Superconducting Magnet,xe2x80x9d issued to Gamble et al., assigned to the assignee of the present invention, and incorporated herein by reference.
The invention features a cryogenic cooling system configured to control the flow of a heat transfer fluid through a remote thermal load, such as a superconducting magnet or rotor.
In a general aspect of the invention, the cryogenic cooling system includes a refrigerator including a cryogenically cooled surface and a cryogenic fluid transport device disposed within the refrigerator for circulating a heat transfer fluid between the cryogenically cooled surface and the remote thermal load.
The cryogenic fluid transport device being positioned within the refrigerator advantageously serves as device for providing the necessary mechanical force necessary to move the heat transfer fluid from the cryogenically cooled surface (e.g., end of a cryocooler) to the remote thermal load. Thus, unlike conventional cooling arrangements the heat transfer fluid does not require a phase change.
Embodiments of this aspect of the invention may include one or more of the flowing features.
The refrigerator is stationary and the remote thermal load rotates relative to the stationary refrigerator. Thus, the system is well suited for use in applications (e.g., rotating machinery, such as motors and generators) where it is difficult to place the heat exchanging cooling device (e.g., cryocooler) in the rotating reference frame where the cooling is needed. The cryocooler can be a Gifford-McMahon cryocooler.
In another aspect of the invention, a method of cooling a rotating thermal load from a refrigerator having a cryogenically cooled surface includes the following steps. A fluid transport device is positioned within the refrigerator. The fluid transport device is operated to provide a heat transfer fluid to the thermal load in an initial non-rotating condition. The thermal load is rotated to a sufficient rotational velocity to generate sufficient forces to cause the heat transfer fluid to move toward the rotating thermal load.
With this approach, operation of the fan can be limited to the initial start up of the rotating thermal load, (e.g., rotor assembly) in a particular mode to increase the reliability of the fan and overall reliability of the system. This approach takes advantage of the difference in density of the cooled heat transfer gas supplied by the refrigerator and the warmer heat transfer gas heated by the rotating thermal load (e.g., superconducting windings) for return to the refrigerator. Specifically, the cold helium being supplied to the rotating thermal load is denser than the warmed gas being returned to the refrigerator. The centrifugal force supplied by the rotating thermal load xe2x80x9cpushesxe2x80x9d the helium radially away from the axis of rotating thermal load. However, because the density of the helium being returned is lower than the helium being supplied, the lower density helium is xe2x80x9cpushedxe2x80x9d toward the axis, thereby setting up a recirculation loop without additional force from the fan.
In embodiments relating to this method, operation of the fan may be terminated after the rotating thermal load has achieved a sufficient rotational velocity.
In certain embodiments of both aspects of the invention, a number of cryocoolers each having a corresponding cryogenically cooled surface may be used to provide a level of redundancy, thereby allowing continued operation of the system in the event that one or more of the cryocoolers requires repair or maintenance. In such embodiments, valving (and appropriate bypass conduits) may be provided to selectively isolate at least one of the plurality of the cryocoolers from remaining ones of the plurality of cryocoolers.
For similar reasons, a number of cryogenic fluid transport devices may be provided to provide the same maintenance and repair advantages described above with respect to the multiple cryocoolers.
Exemplary heat transfer fluids include helium, hydrogen, oxygen, nitrogen, argon, neon, and mixtures thereof.
Cooling system 10 also includes, in this embodiment, a pair of high-speed (10,000-30,000 rpm) fans 21 disposed within the refrigerator 12 for circulating the helium through the cooling system. In essence, fans 21 serve as a mechanical means positioned within the cryogenic environment for providing the necessary force to move the helium past cryocoolers 13 and on to rotor assembly 14. With that in mind, other mechanical devices capable of supplying such forces and operating in a cryogenic environment including diaphragms, piston-operated devices or blowers can serve as fluid transfer device(s) in cooling system 10. Thus, unlike many conventional cooling arrangements the helium (or other cryogenic fluid) need not undergo a phase change to be re-cooled after being heated by the load. As was the case with the multiple cryocoolers 13, a pair of fans 21 is used to provide redundancy and facilitate maintenance in the event that one of the fans requires maintenance or replacement. Of course, appropriate valve and bypass conduits are required to allow each of fans 21 to be isolated from the other while allowing continuous operation of the system. A fan determined well-suited for operation in a cryogenic environment is a Model A20 fan, available from Stirling Cryogenics and Refrigeration BV, The Netherlands.
Operation of fan 21 can be limited to initial start up of rotor assembly 14 in a particular mode to increase the reliability of the fan and overall reliability of the system. In particular, rotor assembly is configured such that the helium gas is introduced through a first introduction line 27 lying substantially along an axis 25 of the rotor assembly. The cooled helium from refrigerator 12 is then provided to the superconducting field windings along a supply line 29 that extends radially away from axis 25. The helium gas is then returned along a first return line 31 which extends radially back to a second return line 33 lying substantially along axis 25 and back to refrigerator 12. Configuring the introduction, supply and return lines in this manner advantageously allows for centrifugal forces generated by the rotating machine to assist and, in certain embodiments, supply all of the necessary force to maintain circulation of the helium between rotor assembly 14 and refrigerator 12. That is, in certain applications, fan 21 may be operated in a more limited fashion to provide supplemental force to the helium flowing through the system, while, in other applications, the fan may only be required to operate at an initial mode in which the motor is not yet rotating.
This approach takes advantage of the difference in density of the cooled heat transfer gas supplied by the refrigerator and the warmer heat transfer gas heated by the superconducting windings for return to the refrigerator. Specifically, the cold helium moving away from axis 25 along supply line 25 is denser than the warmed gas being returned along return line 31. The centrifugal force supplied by the rotating rotor assembly 14, in essence, xe2x80x9cpushesxe2x80x9d the helium away from axis 25 of rotor assembly 14. However, because the density of the helium being returned is lower than the helium being supplied, the lower density helium is xe2x80x9cpushedxe2x80x9d toward the axis, thereby setting up a recirculation loop without additional force from fan 21.
The approaches described above for cooling rotating thermal loads is particularly well suited for HTS superconducting rotating machines, such as those described in co-pending applications, Ser. No. 09/416,626, entitled xe2x80x9cSuperconducting Rotating Machinesxe2x80x9d, filed Oct. 12, 1999 and Ser. No. 60/266,319, entitled xe2x80x9cHTS Superconducting Rotating Machinexe2x80x9d, filed Jan. 11, 2000, both of which are incorporated by reference. The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.